Southern California -- this just in

John Bryson case: Unlike with alcohol, no clear-cut rule for Ambien

The maker of the popular sleeping aid Ambien warns users not to take the drug and drive. But whether it's against the law to get behind the wheel with the sleep-inducer in your system is determined on a case-by-case basis, experts say.

On Tuesday, prosecutors announced that former Commerce Secretary John Bryson would not be charged in connection with last month's bizarre hit-and-run crashes in the San Gabriel Valley, despite his blood work testing positive for Ambien.

Unlike alcohol, California law does not have a specific threshold for how much Ambien can be in one's system while driving. Instead, authorities use a more subjective approach, said attorney and former prosecutor Neil Shouse.

Rather than a clear-cut guideline -– such as the 0.08% for alcohol -– authorities take into account a number of factors, including the driver’s sensitivity to the sleeping aid, how much of the medication is in the system and how apparent the side effects were at the time of the crash, Shouse said.

"It’s often a situation that it has a presence in the blood but not enough to have an intoxicating effect," Shouse said, adding that trace amounts can stay in the blood for days.

In Bryson's blood work, doctors found amounts at the "low end of therapeutic levels," according to a prosecutor's memo, and a criminalist could not say whether it was a factor in the collisions.

The drug's half-life is two hours, meaning half the dose is metabolized and no longer appears in blood tests, said Alon Avidan, director of the UCLA Sleep Disorders Center. Avidan said however that he had no data on when the drug leaves the system entirely.

Shouse said he knew of no states that have a criminal threshold for Ambien in the blood, but that some have advocated that creating one would make "sleep driving" cases easier to prosecute.